1. Field of the Invention
This invention relates to the measurement of fluid flows, particularly in the measurement of wind velocities, wind shears and wind turbulence perpendicular to a line-of-sight.
2. Description of Related Art
This invention relates generally to the art of optical flow diagnostics and more particularly to the art of measurement of a wind profile along an optical path.
As early as 1897, it was reported that the range and direction, and speed of atmospheric currents could be inferred from stellar scintillation in the focal plane of a telescope though no specific method was proposed or demonstrated. It is well-known that scintillation arises from turbulence in the atmosphere.
Refinements of the principle are well-known. These include measurement of the spatio-temporal and spatio-angular correlation function of the scintillation. Simple spatio-temporal correlation is well known to provide overall wind speed and direction, but does not provide resolution of the wind profile along the optical path. Previous attempts to improve resolution have resulted in numerically intractable formulae with poor noise properties. [See D. L. Fried, "Remote Probing of the Optical Strength of Atmospheric Turbulence and Wind Velocity", Proceedings of the IEEE, Vol. 57 No 4, pp. 415-420 (1969)]. The spatio-angular correlation function can be used to measure the range of turbulence, but provides no information on the wind speed. Furthermore, these techniques require that significant averaging be used, and that two sources be used for spario-angular correlation.
Also well known are additional refinements to the basic principle. In one example the average log-amplitude variance was measured at one or more spatial frequencies and weighted combinations of these measurements were used to improve range resolution of turbulence measurement. In such discussions, no reference is made of the possibility of utilizing the same approach for the measurement of wind speed or direction. Additionally, such measurements used sinusoidal spatial filters in the pupil plane which were introduced mechanically and in sequence.
In another well-known refinement, an active source is-used that moves with substantial and known velocity across the field-of-view of the optical receiver. This technique also uses spatial filters in the pupil plane, or the synthesis of signals from many apertures.
In yet another well-known refinement, radar is used in conjunction with the measurement of scintillation. In this technique the wind is measured directly with radar in order to determine the profile of the strength of turbulence along the optical path.
In a fourth well-known refinement, two spatial filters are used in or near the pupil plane, and the light is focused onto a single-element detector which records the filtered, time varying signal. The spatial periods and locations of the two filters are claimed to determine the range of the turbulence from which the scintillation arises, and the spatial frequency of the scintillating turbulence. Given the spatial frequency of the turbulence and the temporal frequency of the recorded signal, the speed of the turbulent layer may be deduced. Evidently, practical implementation of this technique for profiling of wind requires sequential measurements with a variety of spatial filters and filter separations; furthermore the signal is heavily-filtered, resulting in reduced signal and accuracy.
A fifth well-known technique utilizes simple spatial filtering and some defocus from the focal plane. The analysis was not extended to the treatment of extended turbulence, and was not applied to the estimation of the wind profile along the optical path. It was concluded that the approach works only if two spatial filters are provided or if one spatial filter is provided and a sinusoidal source is also available.
Various means available for detecting air turbulence require one or more active laser sources. These techniques are often interferometric in nature and require substantial laser power and coherence.
It is well-known that techniques utilizing Doppler backscatter from radar or lasers can be used to measure wind parallel to the line-of-sight. Acoustic means are also well-known to be able to measure winds along the direction of propagation of the optical wave, and require both a means for transmission and reception of acoustic signals.
It is also well-known that radar can be used to measure winds transverse to the line of sight, though the signal-to-noise ratio of such techniques is poor under clear-air conditions, and the radar array is bulky.
It is also well known that a diversity of mechanical means may be used to measure local winds, but no such approach is capable of sensing winds remote from the device.